Recent rapid development of a radio communication technology realizes a variety of products in communication areas such as mobile phone and an information terminal equipped with a variable-directivity antenna. In the radio communication areas, it is highly expected to increase a transmission capacity due to a necessity to handle more complicated and larger data. Many researches and studies have recently been made to attempt an increase of transmission capacity, particularly, by multiplexing various signals of different dimensions, such as, for example, time, space, polarized waves and codes.
Multiplexing with space, in particular, can ideally be made with an adaptive array antenna having a plurality of a nondirectional antennas and a circuit for synthesizing vectors of signals from the plurality of nondirectional antennas. The adaptive array antenna has inherent disadvantages in a practical usage due to facts that each antenna has a relatively large size and that two adjacent antennas are spaced with a relatively large distance.
An antenna is expected to be as small as possible, especially, in a mobile application area. The variable-directivity antenna is generally made of a pair of an antenna and a power supply circuit and is capable of varying directivity. Such a variable-directivity antenna is believed to be made in a size smaller than the adaptive array antenna and is therefore expected to be a promising candidate for a compact antenna that realizes multiplexing with space. However, only a few studies have been announced on a compact variable-directivity antenna device.
FIG. 1 illustrates an oblique perspective view of a known variable-directivity antenna device. The variable-directivity antenna includes an antenna element 101, a reflecting element 102 and a reflecting element moving means 103. The reflecting element 102 is arranged in parallel to the antenna element 101. The reflecting element moving means 103 includes a rotation drive portion 103a and a connection arm 103b so that the reflecting element 102 moves along a circular-arc around an axial line of the antenna element 101. The reflecting element 102 is arranged perpendicularly via an insulator (not shown) on the rotation drive portion 103a. 
The rotation drive portion 103a is attached on a conductor 104, for example, a car body. Further, the reflecting element 102 is connected via the reflecting element moving means 103. A coaxial line 105 connects electrically the antenna element 101 to a power supply 106. Therefore, the antenna element 101 directs the directivity in a specific direction by adjusting a positional relationship between the antenna element 101 and the reflecting element 102. However, a size of the antenna device is large due to an installation of the reflecting element 102.
FIG. 2 illustrates an oblique perspective view of another known variable-directivity antenna device. The variable-directivity antenna device includes a circular ground plate 111, a single center monopole 112 and parasitic elements 113. The parasitic elements 113 are arranged to surround the single center monopole 112. An impedance load 114 is arranged under a part of the parasitic element 113. A directivity of the antenna device is changed by changing a state of the impedance of the parasitic element 114. However, an interval between the single center monopole 112 and the parasitic element 113 is limited to be λ/4. As a result, the antenna size becomes large and a whole size of the antenna device is more than 2λ.
FIG. 3 illustrates an oblique perspective view of another known variable-directivity antenna device 115. The variable-directivity antenna 115 includes a radiating element A0 and variable reactance elements A1 to A6 and a circular ground plate 116. A radio signal is fed to the radiating element A0. The variable reactance elements A1 to A6 are radially arranged to surround the radiating element A0. However, an interval d between the radiating element A0 and the variable reactance elements A1 to A6 is to be λ/4. As a result, a size of the antenna device 115 becomes large and is more than λ. As described, the proposed variable-directivity antenna devices are larger than the non-directional antenna device.
FIG. 4A illustrates a cross-sectional view of another known variable-directivity antenna device 120. FIG. 4B illustrates a top view of a part of the known antenna device 120 of FIG. 4A. The antenna device 120 is a disk-corn-shaped antenna having a radiating element 121 and a ground plate 123. The antenna device 120 is a non-directional antenna to which an electromagnetic power is fed by a coaxial line 124.
FIG. 5 illustrates a return loss characteristic of the variable-directivity antenna device 120 of FIG. 4A. Similar values of the return loss are obtained in a wide range independent of the existence of the short circuit. However, the return loss is increased in a range below a frequency of 10 GHz. An inductance due to the short circuit is increasing perpendicular to the increase of the frequency. However, the inductance in the range of the frequency of 10 GHz is not large enough to affect an inductance of the antenna device.
FIG. 6 illustrates a cross-sectional view of another known variable-directivity antenna device 130. The antenna device 130 includes a coaxial line 134, an inner conductor 134a, an outer conductor 134b, short-circuit 131, switches 133 and a capacitor 135 on a ground plate 137. The short-circuit line 131 shorts the inner conductor 134a and the outer conductor 134b of the coaxial line 134. The switch 133 switches a state between a shorted state and a non-shorted state. In the antenna device 130, wirings are eliminated using flip chip methodology in an assembly process so as to improve accuracy with less difference among antenna devices.
FIG. 7 illustrates a cross-sectional view of the variable-directivity antenna device 130 of FIG. 6 with wirings to make a short-circuit in a shorted state. FIG. 8 illustrates a cross-sectional view of the variable-directivity antenna device 130 of FIG. 6 with no wiring.
There is a need for a variable-directivity antenna having a wide operating range with a similar size to a non-directional antenna device.